Most coal seams in China have characteristics including high gas pressure, high gas content, low permeability, and strong absorptivity, and it is very difficult to extract gas from the coal seams. Therefore, it is an important approach to improve permeability manually for the coal seams to improve air permeability of the coal seams and improve the gas pre-extraction rate, in order to ensure safe production in the coal mines.
At present, hydraulic measures have been widely applied in the gas control process in the coal mining fields in China, owing to their efficient pressure relief and permeability improvement effect. However, hydraulic measures still have drawbacks such as limited fracturing capability of jet flow impact, high water demand, water accumulation in roadways, and high requirement for borehole sealing, etc.; consequently, the scope of influence of a single borehole is limited, the construction load of boreholes is still not decreased significantly, and the requirement for intensive coal mining can't be met.
A gas flowing at a high speed has characteristics including high compressibility. When a high-energy gas is released instantaneously, the gas will expand and release great energy. However, when a high-energy gas directly impacts a coal mass, the coal mass can be fractured only if the impact strength reaches the compression strength of the coal mass. Consequently, the fracturing effect of direct gas impact is not remarkable.
Relevant researches have demonstrated that the gas absorptivity of a coal mass decreases by about 8% whenever the temperature increases by 1° C. In recent years, many researchers have put forward heat injection-based coal seam gas extraction techniques, which increase the temperature of a coal mass by injecting high-temperature stream into a coal seam, and thereby promote gas desorption. However, owing to the fact that the heat-conduction coefficient of coal mass is not high and the heat injection form is simple, the engineering application effect of these heat injection-based coal seam gas extraction techniques is not remarkable.